Okay, so I got normal boot working again. Sort of.
1. Jumped into edit mode in the GRUB boot menu by pressing the escape key. Selected the 3.8.5-desktop-1.mga3 entry and edited it. Deleted the splash and quiet entries as well as the resume=UUID entry.
2. Boot got as far as VirtualBox guest additions. Did an ACPI shutdown from the VirtualBox window's menu.
3. Edited the 3.8.5-desktop-1.mga3 entry to start in safe mode. Once there, I did
(a) urpme -a virtualbox
(b) urpme --auto-orphans
4. Selected normal boot for the 3.8.5-desktop-1.mga3 kernel and normal boot was successful.
5. Rebooted and selected 3.8.6-desktop-2.mga3 kernel. Again, normal boot was successful.
6. Because I had no network connection in safe mode, I immediately started the Mageia Control Center and selected Software Management > Update your system. There, I found another kernel and vboxadditions update waiting, among others, because I had the kernel-desktop-latest and vboxadditions-kernel-desktop-latest packages installed. Took a chance and installed all updates.
7. Next reboot was from kernel 3.8.7-desktop-1.mga3. I booted normally, and the boot was successful. I now had three different kernels installed and all were booting normally again. However, I couldn't figure out
why my mouse pointer was integrated with both the host and the guest.
8. Started looking in the package list and discovered there are two sets of VirtualBox packages, both metapackages and actual packages. One set is named
vbox and the other is named
virtualbox. I wasn't sure (I'm
still not sure) which set I should have installed.
9. In order to try and sort things out, I uninstalled
all vbox and virtualbox packages. Then I uninstalled the two older kernels.
10. Rebooted normally to the newest (3.8.7-desktop-1.mga3) kernel, now the only one installed. So far, so good. Started the package manager and selected the virtualbox-guest-additions package for installation. It required some dependencies which I marked for installation, and installed all marked packages.
11. Next normal reboot failed. It was hanging at the VirtualBox guest additions message again.
12. Rebooted to safe mode. Did:
(a) urpme -a virtualbox
(b) urpme --auto-orphans
13. Normal reboot worked again. Still had integrated mouse pointer with both the host and the guest.
14. Started the package manager. The dependency packages from installing the virtualbox-guest-additions package in step #10 were still installed. The dependency list is:
vboxadditions-kernel-3.8.7-desktop-1.mga3-4.2.12.mga3.i586
vboxadditions-kernel-desktop-latest-4.2.12.mga3.i586
X11-driver-video-vboxvideo-4.2.12.mga3.i586
xrandr-1.4.0-1.mga.i586
(see screenshot below)

Note that all dependency packages are named
vbox, not
virtualbox, and that I do not have the i586 kernel installed. The installed kernel is 3.8.7-desktop-1.mga3, which is a 686 kernel.
To summarize:
(a) Installing the
virtualbox-guest-additions package marked several
vbox packages for installation.
(b) Those
vbox packages were evidently intended for use with a 586 kernel, which I don't have installed.
(c) Installing the
virtualbox-guest-additions package renders the guest VM unbootable, except in safe mode.
(d) Uninstalling the
virtualbox-guest-additions package still leaves the
vbox packages marked as dependencies installed, even after running urpme --auto-orphans.
(e) Although the guest additions packages intended for a 586 kernel still works with a 686 kernel, there is some jerkiness with the mouse while in the guest VM.
I believe this is a dependency problem, as this situation never arose before now. I'm also still unsure of
which packages, exactly,
should now be installed for the kernel and VM I am running.
HELP
====Can/will someone who knows more than me about Mageia's workings tell me:
(a) which packages I should have installed, and
(b) where I can find the package installation logs?
I am more than happy to do any further troubleshooting, posting of logs or anything else someone may want done in relation to this problem.